1. Field of the Invention
The invention pertains generally to optical systems, and particularly to optical fiber systems.
2. Art Background
Optical systems employing optical fibers as transmission media are now in wide use or have been proposed for a wide variety of uses, including communication, sensing and optical power transmission. These systems typically include a source of electromagnetic radiation, e.g., a laser, as well as an optical fiber which serves to transmit at least a portion of the radiation emitted by the source to a body upon which the radiation is to be impinged, e.g., an optical detector.
A significant factor in the design of optical fiber systems is the optical power loss produced by the optical fiber. This loss is usually measured by relating input optical power, P.sub.i, to output optical power, P.sub.o, via the relation EQU P.sub.o /P.sub.i =10.sup.-.alpha.L/10, (1)
where .alpha. is the loss coefficient of the fiber in decibels per unit length, e.g., per kilometer, of fiber (dB/km), and L is the length of the fiber in corresponding length units.
In the case of, for example, optical fiber communication systems, the optical power loss produced by the fiber attenuates the optical signals transmitted by the fiber. As a consequence, devices, called repeaters, are positioned at regular intervals along the length of the fiber to regenerate the attenuated optical signals. Significantly, the loss coefficient, .alpha., of the fiber largely determines the maximum spacing between the repeaters. At present, the fibers employed in such systems are highly purified silica (SiO.sub.2) glass fibers which exhibit a minimum loss of about 0.2 dB/km at a wavelength of about 1.55 micrometers (.mu.m). This value of minimum loss limits the maximum distance between the repeaters to no more than about 100 km.
In the hope of achieving lower optical losses, and thus, for example, larger repeater spacings, optical systems have been proposed employing multicomponent halide glass fibers. (A multicomponent halide glass is a glass derived, for example, from a melt having constituents which include two or more halides.) These proposals are based on the belief that multicomponent halide glasses exhibit minima in their intrinsic losses (losses due to factors other than impurities, compositional variations and defects), at wavelengths between about 2 .mu.m and about 10 .mu.m, which are far lower than the minimum loss exhibited by silica glass.
Present-day manufacturing techniques have resulted in multicomponent halide glasses having impurity levels which produce relatively large optical losses in various wavelength regions, including the very wavelength regions of the electromagnetic spectrum where the multicomponent halide glasses are expected to achieve minimum intrinsic losses. As a consequence, the minimum intrinsic loss (vacuum) wavelengths, .lambda..sub.min, and the corresponding loss coefficients, .alpha..sub.min, for these glasses are obscured i.e., are not (presently) directly measurable.
In the expectation that new manufacturing and purification techniques will shortly eliminate the unwanted impurities (and defects), optical system designers are even now designing optical fiber systems employing multicomponent halide glass fibers. These designs are based on values of .lambda..sub.min and .alpha..sub.min derived using extrapolation procedures originally developed for, and applicable to, single-component glasses. That is, as depicted in FIG. 1, which includes a semi-log curve of .alpha.versus 1/.lambda. for an ideally pure, defect-free, single-component glass body, the total (intrinsic) optical loss for such a body is the sum of three contributions. The first of these involves absorption of incident photons by valence band electrons, which promotes the electrons into the conduction band (the energies of the absorbed photons are substantially equal to or greater than the energy gap between the valence and conduction bands). This absorption is typically resonant (maximum) in the ultraviolet region of the spectrum, tailing off into the visible and near-infrared regions in an exponential fashion known as the Urbach edge (see FIG. 1). As a result, EQU .alpha.(Urbach)=Ce.sup.c/.lambda., tm (2)
where C and c are positive quantities, approximately independent of the (vacuum) wavelength, .lambda.. (Regarding the Urbach edge see, e.g., V. Sa-Yakanit et al, Comments on Condensed Matter Physics, Vol. 13, pp. 35-48 (1987).)
The second contribution to intrinsic loss is due to absorptions which excite polar optic-phonons, i.e., ionic vibrations involving the creation of electric dipoles. This second type of absorption is typically resonant in the far-infrared wavelength region, tailing off into the near-infrared and visible region in an essentially exponential fashion referred to as the multiphonon edge (see FIG. 1). Thus, EQU .alpha.(Multiphonon)=Ae.sup.-a/.lambda., (3)
where A and a are positive, essentially .lambda.-independent, material parameters. (Regarding multiphonons see H. G. Lipson et al, Physical Review B, Vol 13, pp. 2614-2619 (1976).)
The third contribution to intrinsic loss is due to light scattering by refractive index variations inherent in the material, including: (1) propagating refractive index variations generated by acoustic phonons (ion vibrational excitations manifesting sound waves); (2) propagating refractive index variations generated by optic-phonons; and (3) static refractive index variations due to density fluctuations which were in diffusive thermal equilibrium in the melt but became frozen into the glass on vitrification. Although the scattering mechanisms in (1) and (2) produce scattered radiation which is shifted in wavelength from the incident (vacuum) wavelength, .lambda., the shifts are small (compared to .lambda.). As a consequence, the loss coefficient associated with the sum of the three intrinsic scattering mechanisms (this sum being depicted in FIG. 1) is well approximated by (the so-called Rayleigh form) EQU .alpha.(scattering)=B/.lambda..sup.4, (4)
where B is a material parameter, independent of .lambda.. (Regarding this scattering loss see M. E. Lines, Journal of Applied Physics, Vol. 55, pp. 4052-4057 and 4058-4063 (1984).)
From Eqs. (2)-(4), it follows that the loss coefficient, .alpha., for an ideally pure, defect-free, single-component glass is given by EQU .alpha.=Ae.sup.-a/.lambda. +B/.lambda..sup.4 +Ce.sup.c/.lambda.. ( 5)
As shown in FIG. 1, the three terms in Eq. (5) define an optic window of relatively low attenuation, i.e., a range of wavelengths where .alpha. is relatively small. Significantly, the absolute minimum loss coefficient, .alpha..sub.min, is positioned within the optic window at a vacuum wavelength, .lambda..sub.min, where the dominant loss involves (Rayleigh) scattering and multiphonons, i.e., the Urbach term in Eq. (5) is negligibly small. Consequently, for purposes of predicting .lambda..sub.min and .alpha..sub.min, the intrinsic loss coefficient, .alpha., is well approximated by the sum of the multiphonon and scattering terms only, i.e., EQU .alpha.=Ae.sup.-a/.lambda. +B/.lambda..sup.4. (6)
Differentiating Eq. (6) with respect to .lambda. and equating the result to zero yields .lambda..sub.min and .alpha..sub.min as the solutions to EQU .lambda..sup.3.sub.min exp (-a/.lambda..sub.min)=4B/Aa (7) EQU and EQU .alpha..sub.min =(B/.lambda..sup.4.sub.min)(1+4.lambda..sub.min /a). (8)
When estimating values of .lambda..sub.min and .alpha..sub.min for multicomponent halide glasses, using semi-log curves of measured values of .alpha. versus .lambda..sup.-1 (where .lambda..sub.min and .alpha..sub.min are obscured by extrinsic losses), optical system designers have employed techniques which exactly parallel those applicable to single-component glasses. That is, these designers have assumed that a single exponential curve, and thus a single value of the exponent a, is sufficient to define the multiphonon edge for any multicomponent halide glass both in the unobscured and obscured regions of the measured .alpha. versus .lambda..sup.-1 curve. Based upon this assumption, these designers have fitted a single exponential to the unobscured data, and have extended this exponential into the obscured region. In addition, by using light scattering techniques to define the value of the parameter B, system designers have similarly extended the portion of the loss curve clearly attributable to (Rayleigh) scattering into the obscured region. The point of intersection of these two curves has been used to define .lambda..sub.min and .alpha..sub.min, which is substantially equivalent to the differentiation procedure, described above.
Thus, those engaged in the design and development of optical fiber systems employing multicomponent halide glass fibers have sought, and continue to seek, improved techniques for determining .lambda..sub.min and .alpha..sub.min.